1. Field of the Invention
The present invention relates to a biological substance involved in insulin resistance, and more specifically, to an insulin resistance marker, a method of evaluating insulin resistance, a method of screening a substance that improves insulin resistance, and a pharmaceutical composition for improving insulin resistance. More specifically, the present invention relates to prevention of occurrence, diagnosis and therapy of insulin resistance and disease conditions and states accompanied with insulin resistance.
2. Disclosure of the Related Art
Now in Japan, the number of patients suffering from metabolic syndromes such as diabetes, hypertension, and hyperlipidemia that will cause severe adult diseases is dramatically increasing, and development of methods to prevent, diagnose and treat them is needed.
Since the major cause of a metabolic syndrome is obesity, various researches are undertaken to elucidate the mechanism of obesity. The 3T3-L1-strained cell derived from a mouse is one of the cells that are most frequently used in such researches as a model of an adipose tissue. A 3T3-L1 cell is proved to have a property that is well coincident with biochemical and physiological characteristics in an actual adipose tissue in such a meaning that by induction of differentiation, fat droplets are accumulated in the cell and uptake of extracellular glucose via an insulin receptor is promoted (Non-patent Document 1: Cell, Vol. 5, 19-27, May 1975).
As a research example using a 3T3-L1 cell, concretely, Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2004-105175 reports on a protein and a gene secreted by an adipocyte, Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2004-135605 reports on a protein and a gene involved in differentiation of an adipocyte, Patent Document 3: Japanese Patent Application Laid-Open Publication No. 2005-247740 reports on a protein having a function of controlling fatty acid metabolism that is expressed in the course of differentiation of an adipocyte, and Patent Document 4: Japanese Patent Application Laid-Open Publication No. 2006-141233 reports on a secreted protein and a gene involved in adipocyte hypertrophy.
3T3-L1 adipocytes are also used as an in vitro experimental model of human adipose tissues. For example, PPARγ (peroxisome proliferator-activated receptor γ), a key molecule for proliferation and differentiation of adipose cells, is known to have similar functions in 3T3-L1 adipocytes as well. Further, it was demonstrated that functions and expression patterns of many proteins in vitro experimental models were reflected in those in vivo such as mouse and human adipose tissues. Concrete examples of such a protein include adiponectin (Non-patent Document 2: Biochemical and Biophysical Research Communications 290, 1084-1089 (2002), Non-patent Document 3: NATURE MEDICINE, VOLUME 7, NUMBER 8, 941-936, AUGUST 2001, Patent Document 5: International Publication No. 2003/063394 pamphlet, Patent Document 6: International Publication No. 2004/061108 pamphlet), resistin (Non-patent Document 4: NATURE, VOL 409, 13, 292-293, JANUARY 2001, Non-patent Document 5: Biochemical and Biophysical Research Communications 288, 1027-1031 (2001), Patent Document 5, Patent Document 6), free fatty acid (Non-patent Document 6: THE JOURNAL OF BIOLOGICAL CHEMISTRY VOL. 280, NO. 42, pp. 35361-35371, Oct. 21, 2005), and visfatin (Non-patent Document 7: Biochemical and Biophysical Research Communications 359 (2007) 194-201) and the like.
Obesity, which results from adipocyte hypertrophy, is one of the primary risk factor of metabolic syndrome and insulin resistance in physiological and clinical implications. An adipose tissue is known as an important endocrine organ that regulates whole-body insulin sensitivity. Currently, a number of adipocytokines having an influence on differentiation and proliferation and hypertrophy of fat, and insulin sensitivity (for example, the aforementioned adiponectin, leptin, tumor necrosis factor α (TNFα) and so on) have been identified.
Insulin resistance is fundamental to the pathogenic factor of metabolic syndromes including type 2 diabetes mellitus. Therefore, it is expected that diagnosis and improvement of insulin resistance will lead to prevention and fundamental therapy of the lifestyle-related diseases including diabetes.
However, it is actually the case that a metabolic syndrome is detected by health examination or the like only after abnormality occurs in blood pressure or lipid metabolism following onset of insulin resistance. In other words, it is, in fact, very difficult to diagnose onset of insulin resistance in an early stage.
Therefore, discovery of a novel diagnostic marker capable of specifically detecting insulin resistance is demanded.
The aforementioned adipocytokines have not been brought into practical uses for therapy and diagnosis. On the other hand, as an insulin sensitizer for diabetes, thiazolidine-based drugs targeting an intranuclear receptor PPARγ are currently representative. Thiazolidine-based drugs have the effect of normalizing regulation of glycometabolism in the body by improving the insulin sensitivity. However, as for these drugs, problems such as side effects like hepatic dysfunction and necessity of strict drug administration control have been indicated.
Therefore, discovery of a novel drug target that shows a specific action on insulin resistance is demanded.
On the other hand, insulin resistance is known to be developed not only by environmental factors such as obesity and daily habits but also by various clinical factors such as pregnancy and long-term dosing of steroids. Although the reason why insulin resistance is induced due to such varied factors is not known, there is a report that these phenomena are also observed in in vivo and in vitro experiments. In other words, it was reported that a glucocorticoid (for example, dexamethasone) etc., in addition to an obesity-related factor (TNFα), induces insulin resistance of 3T3-L1 adipocytes (Non-patent Document 9: DIABETES, VOL. 49, 1700-1708, OCTOBER 2000, Non-patent Document 8: Proc. Natl. Acad. Sci. USA, Vol. 91, pp. 4854-4858, May 1994).
As for molecular mechanisms of insulin signal transduction, generalization has been widely made (Non-patent Document 10: Mol Cell Biochem 182, 31-48 (1998), Non-patent Document 11: Diabetes Metab 24, 477-89 (1998), Non-patent Document 12: Cell 92, 593-6 (1998), Non-patent Document 13: J Clin Invest 103, 931-43 (1999)).
Specifically, the interaction between insulin and insulin receptor results in activation of tyrosine kinase and phosphorylation of IRS-1. Then PI3-kinase (phosphoinositide 3-kinase), Grb2.Sos complex, and SHP-2 bind to the phosphorylated IRS-1. As a result, PI3-kinase is activated, and interacts with Akt (also referred to as PKB, protein kinase B). Akt is phosphorylated and activated by PDK1 (phosphatidylinositol-dependent protein kinase 1). The activated Akt is dissociated with the plasma membrane and phosphorylates various proteins. Therefore, in muscle cells (skeletal muscle, myocardium) and adipocytes, translocation of glucose transportors and GLUT4 (glucose transporter 4) containing vesicles are promoted. Thus, cellular uptake of glucose is promoted.
By the way, a proepithelin protein (also known as progranulin/PCDGF/PEPI/GEP/GP88) which is an epithelin precursor protein having seven kinds of epithelin domains is known. It is reported that proepithelin and epithelin have a growth factor-like function, and function as an autocrine growth factor as a result of being secreted outside the cell, and thus are involved in inflammation and cell migration (Non-patent Document 14: Journal of Molecular Medicine 81, 600-612 (2003)).
However, a relationship between such a protein or a peptide and insulin resistance is not known at all.                Patent Document 1: Japanese Patent Application Laid-Open Publication No. 2004-105175        Patent Document 2: Japanese Patent Application Laid-Open Publication No. 2004-135605        Patent Document 3: Japanese Patent Application Laid-Open Publication No. 2005-247740        Patent Document 4: Japanese Patent Application Laid-Open Publication No. 2006-141233        Patent Document 5: International Publication No. 2003/063894 pamphlet        Patent Document 6: International Publication No. 2004/061108 pamphlet        Non-patent Document 1: Cell, Vol. 5, 19-27, May 1975        Non-patent Document 2: Biochemical and Biophysical Research Communications 290, 1034-1089 (2002)        Non-patent Document 3: NATURE MEDICINE, VOLUME 7, NUMBER 8, 941-946, AUGUST 2001        Non-patent Document 4: NATURE, VOL 409, 18, 292-293, JANUARY 2001        Non-patent Document 5: Biochemical and Biophysical Research Communications 288, 1027-1031 (2001)        Non-patent Document 6: THE JOURNAL OF BIOLOGICAL CHEMISTRY VOL. 280, NO. 42, pp. 35361-35371, Oct. 21, 2005        Non-patent Document 7: Biochemical and Biophysical Research Communications 359 (2007) 194-201        Non-patent Document 8: Proc. Natl. Acad. Sci. USA, Vol. 91, pp. 4854-4858, May 1994        Non-patent Document 9: DIABETES, VOL. 49, 1700-1708, OCTOBER 2000        Non-patent Document 10: Mol Cell Biochem 182, 31-48 (1998)        Non-patent Document 11: Diabetes Metab 24, 477-89 (1998)        Non-patent Document 12: Cell 92, 593-6 (1998)        Non-patent Document 13: J Clin Invest 103, 931-43 (1999)        Non-patent Document 14: Journal of Molecular Medicine 81, 600-612 (2003)        